This invention relates to a colour picture tube that is constructed using a shadow mask, frame, inner shield and bimetal and other tube elements with good formability and excellent thermal characteristics and gives high displayed image quality.
A colour picture tube has electron guns, e.g., in an in-line array, in a neck portion formed at one end of a glass envelope and red, blue and green phosphors provided in an image-division array on a face portion at the other end of the glass envelope facing these electron guns. A shadow mask with a plurality of beam holes is placed near to and facing this fluorescent surface. As this shadow mask is made integral with a frame by welding around its periphery and the frame is mounted on a face portion via attachment elements including bimetal, the frame is further provided with an inner shield to shield against the effects of geomagnetism.
In a colour picture tube thus constructed, electron beams emitted by the electron guns are deflected by deflection control effected by a deflection device provided at the root portion of the neck portion and pass through holes in the shadow mask to strike the fluorescent surface and cause fluorescence and so define a coloured picture.
In the past, the materials used for shadow masks, frames and inner shields have been rimmed steel and Al killed steel, etc., which have good etchability and formability and on whose surfaces there is easily formed an oxide film that contributes to reduction of electron beam reflection. Recently, however, there have been demands for higher picture tube quality, i.e., for so-called good viewability and fine detail of displayed images, in order to meet the needs of new media and there have been found to be drawbacks in use of shadow masks, frames and inner shields that are made of rimmed steel and Al killed steel as noted above.
In more detail, the temperature of these elements rises to 30.degree.-100.degree. C. during operation of a colour picture tube and, for example, so-called doming occurs because of distortion of the formed shape of the shadow mask caused by its thermal expansion. As a result, misalignment in the positional relationship of the shadow mask and fluorescent screen occurs, so giving rise to colour fringing, or purity drift (PD). In high definition colour picture tubes in particular, there is a proportionally greater amount of misalignment since the shadow mask hole diameter and hole pitch are very small and tube elements as described above for which the material used is rimmed steel or Al killed steel become unserviceable for practical purposes. This problem is particularly apparent in high radius of curvature picture tubes in which the face portion and shadow mask are brought close to a plane in order to reduce image distortion and reflection of external light.
Use of Ni-Fe alloys, e.g., invar (36 Ni-Fe), with a small coefficient of thermal expansion as material for these tube parts or elements has been proposed in the past, e.g., in U.S. Pat. No. 4,420,366 (Oka et al.) but the heat conductivity of these Ni-Fe alloys is very poor and as well as there being liable to be accumulation of heat, there is liable to be so-called springback, or inward curving from the spherical surface of an ordinary shadow mask towards the electron gun end. There are also drawbacks in connection with etchability and formability, since when shadow mask holes are formed by etching there is liable to be unevenness of hole diameter, etc. Also, a blackening film formed on the surface as disclosed in Japanese Laid-open Patent Application No. 50-58977 is liable to peel off, so imposing limits on designs to upgrade colour picture tubes.